To control multiple strings of LED lights—for instance for backlighting applications, or for red-green-blue (RGB) colour variable lighting—it is known to provide pulse-width modulation (PWM) signals to a respective switch for each string, to turn that string on and off: the greater the proportion of the cycle the string is turned on (that is, the higher the ‘mark-space ratio’, or duty-cycle of the PWM signal), the brighter the light output from the string. In the example case of RGB strings, the colour of the resultant combined light may be varied by altering the duty cycle of one or more of the PWM signals.
Typically such multi-string LED PWM control schemes use a common, fixed, frequency, often in the range of 200 to 400 Hz. Particularly in the case that the PWM switching for these strings is coincident, this can result in significant electromagnetic interference (EMI) since the switching transients occur at the same frequency and may even be coincident. Further, in applications in which a switch mode power supply (SMPS) is used to provide power, the output stage of the switch mode power supply may be stressed, and may produce audible noise at the PWM switching frequency.
It has been proposed to alleviate the EMI by introducing a random PWM control to the switching, such as is proposed in United States patent application, publication number US 2012/0127210 by Huang et al.